The invention relates to a position controller, particularly for a drive-actuated valve, with an inherent safety design.
European Publication EP 0 637 713 A1 discloses such a position controller for a drive-actuated valve. The valve is installed in a pipe and controls the passage of a medium by way of a corresponding stroke of a closing element that interacts with a valve seat. A pneumatic drive is connected, by a push rod, with the closing element. A lever engages with the push rod and acts on a potentiometer, which functions as a position transmitter of the position controller. The potentiometer detects the actual position of the actuator. A control unit of the position controller compares this actual position with a predefined desired position. As a function of the determined deviation, the control unit generates an actuating signal to control the pneumatic drive. The desired value is predefined for the position controller through a normalized signal, e.g., a 4 to 20 mA interface or a digital field bus message. Thus, the role of the position controller is to convert the predefined desired value of the actuator position into a pneumatic pressure signal that is supplied to the pneumatic drive and results in a corresponding position of the push rod.
In addition, flap valves are known in the art in which the opening angle of a rotary valve is detected by means of a rotary potentiometer. In this case, a position controller generates an actuating signal for a rotary actuator that controls the rotary valve.
Slide potentiometers, because of their simple and inexpensive construction, are frequently used for position detection. Their advantage is that they produce a usable electrical actuating signal in a relatively simple manner with low power consumption. For instance, a 10 kxcexa9 potentiometer operated at 3 V consumes a maximum of 300 xcexcA. The stroke or rotary movement of the actuator is applied to the potentiometer""s axis of movement via corresponding add-on parts, e.g., a rotary lever with a switchable gear drive, and the component voltage detected by the potentiometer is transmitted to the analog input of an analog or digital control unit. The detection range of the angle of rotation for rotary actuators is typically 120xc2x0 maximum. For linear actuators, typically the detection range is 15 mm maximum. The linear motion can also be converted into an angle of rotation of 120xc2x0 maximum by means of a conversion mechanism.
In many areas of process and power technology, the fault-free operation of a plant depends on the flawless functioning of the control valves used. Downtimes of plants or plant parts caused by component failures significantly reduce the production capacity and the possible utilization of the plant. Thus, reducing downtimes and increasing system reliability are essential goals for efficient plant operation.
Due to their construction, the electromechanical slide potentiometers, which are frequently used for rotary or linear position detection, have drawbacks regarding their long-term stability because of wear and oxidation of the contact paths as well as because of their vibration fatigue limit. After prolonged quasi-static operation, their sliders tend to stick. Due to mechanical wear, the sliders and the resistive coatings eventually wear or their quality changes as a result of aging and oxidation. In electromechanical slide potentiometers, the rotary or linear motion is transmitted by means of a continuous shaft. Suitable encapsulation against environmental influences is therefore very costly and in itself is susceptible to aging and wear.
European Patent EP 0 680 614 B1 discloses a device for detecting an angular position of an object. The sensors described in this patent specification are based on the giant magnetoresistive (GMR) effect and consist of alternating magnetically hard and magnetically soft metal layers. These layers are each only a few atoms thick and are sputtered onto a silicon substrate. The resistance of the sensors greatly depends on the direction of a magnetic field acting on them. A GMR sensor is thus very well suited to detect a change in the angular position of a magnet.
Electropneumatic position controllers are frequently used in areas where the atmosphere can change into an explosive gas mixture. Here, it must be ensured that no risk arises from any of the possible operating states of the position controller. For this purpose, the occurrence of an ignition spark must be either excluded by suitable means or ignition of the atmosphere in the environment of the position controller must be reliably prevented.
One object of the invention is to provide a position controller, particularly for a drive-actuated valve, which is distinguished by its improved interference immunity and its inherently safe design, while being inexpensive to produce.
To attain this and other objects, according to the principles of the present invention, and according to one formulation, the a novel position controller, for a valve (2) actuated by a drive (6), having an inherent safety design, includes: a position transmitter (9) that detects the actual position of an actuator (7), and a control unit (13) that compares the actual position with a predefined desired position and that generating an actuating signal, wherein the position transmitter includes a magnet (10) and a magnetoresistive sensor (50), and wherein the magnet and sensor are rotatable or displaceable relative to one another in conjunction with a movement of the actuator (7), wherein the control unit (13) is arranged in a flame-proof enclosure that is separate from a housing of the position transmitter (9), and further wherein an interface (18) of the control unit (13) to the position transmitter (9) as well as the position transmitter (9) itself have an inherent electrical safety design.
The invention has the advantage that an inherently safe design of the position controller is realized at relatively low production costs. High investments for protecting the operating voltage, which would be incurred in a position controller with an inherent electrical safety design not having an ignition-proof enclosure, are avoided due to the flame-proof housing of the control unit. For the position controller, it is sufficient to use an inexpensive plastic housing. There are no increased ignition protection requirements regarding the electrical connection between the control unit and the position controller. The control unit is mechanically and thermally decoupled from the actuator. Mechanical vibrations and shocks, as well as high or low temperatures, of the plant are thus not transmitted to the control unit. With an appropriate encapsulation of the position controller, it is possible to use the unit in a chemically loaded atmosphere or in an explosive environment because of the dual protection achieved. The position controller, which is located outside the housing of the control unit, is advantageously mounted to the actuator even on small drives or under extreme loads that are due, for example, to mechanical vibrations, shocks or extreme temperatures. The position controller then is electrically connected to the control unit at low cost. The control unit is mounted at some distance, e.g., on a mounting tube or a similar mounting aid, from the position controller and then connected thereto by an electrical cable connection, Further the control unit is connected with the pneumatic drive via one or two pneumatic lines.
The invention further obviates the drawbacks of conventional potentiometers, since it uses a contactless potentiometer that comprises a magnet and a magnetoresistive sensor. The novel position transmitter provides the exact actual position of the actuator in either a dynamic or a static case. A non-linearity of the position transmitter""s output signal, which is minor in any case, is readily compensated. Between the magnet and the magnetoresistive sensor, a partition can easily be installed for encapsulation and, thus, protection against environmental influences. Therefore, the position transmitter is very rugged and insensitive to dirt and a harsh environment. The magnet is easily mounted outside the sensor housing on a linear or rotary actuator such that its magnetic field lines act on the magnetoresistive sensor through the housing wall. An evaluation circuit is readily integrated in the sensor housing. This evaluation circuit generates a voltage proportional to the angle of rotation, or the linear path, of the magnet by way of the change in resistance of the magnetoresistive sensor. Thus, the evaluation circuit supplies, to a control unit, a signal that corresponds to the actual position and is immune to interference.
A minimum distance between the magnet and the sensor is easily kept to prevent damage to the magnetically hard layers, especially in a GMR sensor, since in this sensor type the strength of the magnetic field may not exceed 15 kA/m. The contactless principle of the novel position transmitter eliminates the problem of a scratching or sticking slide potentiometer. This contactless principle offers advantages in applications where the potentiometer is exposed to continuous vibrations. It is also advantageous in the quasi-static case where the potentiometer position remains unchanged over a long period of time, and where there is a risk that the slider of a slide potentiometer would dig into the resistance layer and possibly get stuck there due to control instability in the system. If the magnet forms the moving part of the position transmitter, which is coupled with the actuator, it couples the actuating movement into the magnetoresistive sensor through its magnetic field, without requiring any mechanical duct. By corresponding add-on parts, an exact rotary or linear motion of the moving part is ensured in a simple manner.
If the magnet is designed as a permanent magnet, a particularly simple structure results, since the magnet does not require a power supply, and thus does not increase the current consumption of the position transmitter.
An advantageous clear increase in the resistance of the magnetoresistive sensor results if a so-called anisotropic magnetoresistive sensor is used. When the magnetization of the layer is rotated relative to the current direction of a measuring current flowing through the layer of the sensor, there is a change in the resistance in this type of sensor, which can be a few percentage points of the normal isotropic resistance. This ensures a sufficiently high signal-to-noise ratio of the measurement signal.
Using a so-called giant magnetoresistive (GMR) sensor has the advantage that the change in the resistance is independent of the field strength within a wide range, and is only sensitive to the direction of the magnetic field. This directional dependence of the resistance resembles a cosine function, and is therefore nearly linear within a wide range.
Advantageously, the same sensor construction can be used for installation, in both rotary actuators and linear actuators, without requiring any structural changes. For this purpose, the GMR sensor is arranged in the area of the edge of a housing in such a way that the same sensor can be positioned to detect a relative rotational movement at least approximately on the axis of rotation of a magnet that is provided for this case, and to detect a relative shift jointly with a magnet that is provided for this case in a plane that extends substantially perpendicularly to the aforementioned axis of rotation. The distance between the sensor and the housing wall facing the magnet is preferably about 5 mm. This ensures that the required minimum distance between magnet and sensor is met. Since the sensor can be used in both rotary and linear actuators, the costs of logistics and warehousing are reduced because only one GMR sensor type is required.
Improved measurement accuracy in case of temperature fluctuations is obtained by arranging a temperature compensation circuit in the housing of the GMR sensor. To obtain particularly good temperature compensation, the bridge resistance of the GMR sensor is simultaneously used as a measurable resistance for the temperature compensation circuit. This completely eliminates problems of thermal coupling between the measuring resistor and the GMR sensor.
Advantageously, the GMR sensor is arranged on one side and the temperature compensation circuit on the other side of the same printed circuit board. As a result, the components of the temperature compensation circuit, the housing of which is typically larger than the component housing of the GMR sensor, do not need to be arranged between the GMR sensor and the exterior of the position transmitter housing that faces the magnet. Therefore, the components of the temperature compensation circuit do not influence the spacing between the GMR sensor and the exterior of the position transmitter housing. This makes it possible to keep a small distance between the upper edge of the component housing of the GMR sensor and the position-transmitter housing exterior.
Precise positioning of the magnet relative to the GMR sensor can easily be achieved by providing a centering aid on the housing of the GMR sensor to adjust the relative position of the magnet in relation to the sensor during installation. This positioning aid is configured as a molded part that is placed on the magnet, removed again after installation, and that is inserted into an opening on the housing of the GMR sensor in a positive fit during installation. After the magnet and the GMR sensor have been attached, the molded part is removed.
A mechanically positive-locking configuration of the moving part and the sensor housing ensures the spatially correct positioning of the magnet and the sensor. The connections of the two parts to form a complete position transmitter can be non-positive, i.e., wireless. Alternatively, the position transmitter can be constructed as a complete, mechanically integral, position transmitter block, which comprises the moving part with magnet, GMR sensor and evaluation electronics. Such a position transmitter ensures a defined distance between the magnet and the GMR sensor. In principle, active evaluation electronics that are completely isolated from the moving part, both mechanically and electrically, make possible robust and interference-free position transmitter electronics in miniature form that are screened in a simple manner against electrical as well as magnetic interference. The magnet itself requires no mechanical duct through a partition to the housing of the GMR sensor and is located together with the GMR sensor in a common screened chamber for protection against electrostatic and electromagnetic interference. For applications in areas that are subject to extreme interference, a corresponding external screen, which also encloses the magnet, can be constructed as an add-on component if required.
Other advantages of the novel position controller include its: readily integratable protective circuits; wide supply voltage tolerances; minimized external interference through integrated screens and EMI filters; minimized temperature influences with low supply current; minimized and stably reproducible hysteresis as a function of the angle of rotation between magnet and GMR sensor; and field strength.
The minor hysteresis fluctuations and the minor non-linearity, resulting from manufacturing tolerances of the GMR sensors, are irrelevant for the application as a position transmitter in a position controller. If the position controller is intended to output the actual position, as information to additional components of a system, the output signal is easily corrected and actively filtered corresponding to the known linearity and hysteresis characteristics of the individual GMR sensor. For this purpose, if required, the specific correction data of the GMR sensor is stored in a microcontroller of the position controller. For simplified correction of linearity and hysteresis errors, it is sufficient to determine and store five basic sensor-specific characteristic values, which are recorded under standard conditions. These basic values are located, for instance, at the points of the maximum change in the slope of the curves. For a more precise correction, all the characteristic curves can be stored with the desired resolution in a serially readable memory medium supplied with the GMR sensor and assigned to the GMR sensor by an identification key. The content of the memory medium is loaded, for instance, into the microcontroller upon installation of the GMR sensor.